This invention relates to a method for the manufacture of electric motors, and more particularly to a method of induction annealing the rotor cage of squirrel cage rotor assemblies.
Generally, a squirrel cage rotor assembly for a fractional horse power alternating current motor includes a core constructed from a stack of generally circular laminations punched from suitable ferro-magnetic sheet material (e.g., a high magnetic permeability steel). Each of the laminations has a central opening and a plurality of notches arranged around the central opening near its outer margin. These laminations are assembled in a stack so as to form the rotor core with their central openings coaxial so as to form a rotor bore extending longitudinally through the core and with the notches arranged to form a number of slots extending longitudinally through the core. Preferably, the laminations are slightly angularly displaced (skewed) from one another so that the slots are not parallel to the central bore of the core, but rather are wrapped around the core in spiral fashion. The core is then placed in the die of a suitable die casting machine, and an aluminum alloy (preferably electric grade aluminum) is die cast around selected portions of the core. The molten aluminum is forced under pressure through the slots of the core so as to form conductor or rotor bars and so as to form suitable end rings which are cast-in-place on the end faces of the core so as to connect the ends of the conductor bars. In addition, cooling fan blades may optionally be integrally cast with the end rings. The die cast conductor or rotor bars and the end rings of a squirrel cage rotor are oftentimes collectively referred to as a rotor cage.
It has long been observed that by annealing "as cast" squirrel cage rotor assemblies, the conductivity of their rotor cages could be incrementally improved. Of course, the improved conductivity of the rotor cage results in an increase in virtually all of the operating characteristics of and the efficiency of a motor incorporating the rotor assembly. The reason for the annealing process enhancing the conductivity of the rotor assembly has not, to date, been clearly understood. Certain experts in the field have expressed the opinion that the annealing operation stress relieves the rotor cage thus improving its electrical conductivity. It is known that the electrical resistivity of aluminum is increased by cold working. For example, the conductivity of cold drawn aluminum wire is reported to be about 2% less than the same wire when thoroughly annealed (see page 2-27, Electrical Engineers' Handbook, Pender and Del Mar, 4th Edition, John Wiley and Sons, publishers). Others in the motor field have theorized that the improvement in conductivity of annealed rotor assemblies may be accounted for by the fact that upon heating of the rotor assembly in a heat treat furnace, the difference in thermal expansion of the aluminum conductor bars and the steel laminations of the core (e.g., 2.21 v. 0.55 watts cm.sup.2 per degree per cm, respectively) resulted in an electrical separation of the rotor bars from the laminations thus reducing losses due to shorting through the steel laminations. Still others are of the opinion that the improvement in conductivity of the rotor cage observed by annealing the rotor is the result of metallurgical transformations of the aluminum. It is thought that by annealing the "as cast" aluminum rotor cages, certain metallurgical transformations occur (e.g., impurities in the aluminum may precipitate out of solid solution and grain growth is enhanced) which results in increased conductivity.
By the term annealing, it is herein meant that the specimens to be heat treated are held at an elevated temperature for a time sufficient to result in the above-noted increase in electrical conductivity.
Heretofore, squirrel cage rotors were conventionally annealed by placing them in a gas fired heat treat furnace and by allowing them to soak at the prescribed elevated temperature for a predetermined time. However, this process was relatively slow due to the fact that the rotors were heated only by convection and radiation heat transfer within the furnace. With this furnace heat treat method, not only was the die cast aluminum rotor cage heated, but also the laminations of the rotor assembly body which comprise a high percentage of the mass (and hence heat capacity) of the entire rotor body were also heated thus requiring a substantial time for heat treatment of the rotor assembly. Of course, these prior art furnace heat treating operations required a substantial amount of time during the manufacture of rotor assemblies and hence accounted for an appreciable fraction of the cost of manufacturing the rotor assemblies. Also, these prior furnace soak heat treating methods resulted in the wasting of considerable energy due to the inefficiencies of the heating of the rotor assemblies in the furnace and also due to the fact that the rotor core also had to be heated.
In addition to furnace annealing, rotors have heretofore been annealed by various induction heat treating processes. In general, these prior induction rotor heat treating processes utilized either a high frequency induction field (e.g., 1,000-3,000 Hz.) which, in certain instances, necessitated specialized and relatively expensive induction heating equipment, or which required induction heaters including a transformer with a primary winding energized by 60 Hz. line voltage and a water cooled secondary winding coupled to a suitable water cooled coil which heated the rotor assembly. Of course the necessity of providing specialized high frequency induction heating equipment required a substantial capital investment to utilize an induction heating system in the manufacture of rotor assemblies. However, in the prior transformer induction heating arrangements as above described, the requirement of a transformer with a water cooled secondary winding resulted in substantial heating inefficiencies such that the potential heating efficiency of the induction heating process was not realized.